The present invention relates, in general, to chemical bath apparatus and, more particularly, to high temperature acid baths of the type used for etching and stripping semiconductor devices.
In the manufacturer of semiconductor devices it is quite common to employ a manufacturing technique in which semiconductor wafers are etched in an acid bath. The acid bath apparatus commonly employed for this purpose is a relatively rectangular quartz glass vessel or beaker which has a heating coil or blanket-type heater affixed to the outside of the beaker and which is supported and mounted in a surrounding housing or shell. Typical of such prior art devices is the chemical bath apparatus disclosed in U.S. Pat. No. 4,350,553.
Such chemical baths usually employ a highly corrosive acid, such as sulphuric acid, which is heated to a temperature on the order of about 150 degrees C. The heated acid predictably produces highly corrosive fumes which, together with any splash or spillage as a result of inserting or removing semiconductor wafers, can rapidly corrode and destroy the heater mounted to the outside of the beaker. In addition to destruction of the heater, which can be costly, corrosion of the heater can also create extremely hazardous conditions.
Accordingly, the acid bath apparatus of U.S. Pat. No. 4,350,553 includes a structure in which the beaker is supported from the surrounding housing or shell on a seal which resists the passage of fumes and spillage down between the housing and beaker to the heating element. Additionally, the bath apparatus of U.S. Pat. No. 4,350,553 includes a nitrogen purging system which is used to pressurize the chamber between the beaker and the supporting housing so that acid splash and acid vapor do not reach the heating element.
The acid bath apparatus of U.S. Pat. No. 4,350,553 employs a seal between the horizontal extending lip on the acid beaker and the surrounding supporting housing. The entire weight of the beaker and acid contained therein is, therefore, supported by the lip of the beaker. In large acid baths, this weight can be in the order of 50 to 60 pounds, placing a substantial stress on the beaker. Since the beakers which are employed in prior art acid baths have been quartz glass beakers, they inherently tend to be somewhat fragile and susceptible to stress-induced cracking. The problem of stresses in acid bath beakers is further aggravated by heating of the beaker, which induces thermal stress. As will be appreciated, fracture or failure of the beaker lip upon support and heating of in excess of ten liters of sulphuric acid also can result in a very hazardous spillage of the acid.
Another approach which has been taken in connection with heated chemical baths is to fill the space between the support shell and quartz beaker with rock wool. This approach also has been found to have drawbacks, namely, the rock wool is not impervious to acid and it tends to support the beaker unevenly. Accordingly, there can be hard spots in the rock wool which will produce very substantial stress concentrations that will be aggravated upon heating of the beaker. More over, if there is uneven insulation, a hot spot can occur which will degrade the polypropylene shell. The result, again, can be failure of the fragile quartz glass beaker and/or deterioration of the rock wool to a degree which will subject the heater element to corrosive fumes.
Typical of other chemical bath apparatus are the devices of U.S. Pat. Nos. 3,411,999, 3,294,603, 3,095,463 and 1,276,599. Such apparatus, however, employ flange support for the beaker (U.S. Pat. No. 3,411,999), submersed heat transfer means and a complex multi-tank structure (U.S. Pat. No. 3,294,603), a discrete brick liner for a rubber coated steel vessel that is coupled to an external heat exchanger (U.S. Pat. No. 3,095,463), and a simple electrolysis bath (U.S. Pat. No. 1,276,599). Such prior art chemical bath apparatus inherently induce stress concentrations in the glass beaker or are not well suited for use as a semiconductor wafer etching bath.